Check if Table exists in Predicate Filter in SQL Server

Question

I have written a security policy which contains a predicate filter to apply row level security in SQL Server. But, I want to apply the filter predicate only if the table exists in the database. Can anyone please suggest anyway I can to it? Below is the security policy:

IF OBJECT_ID('s.policyName') IS NOT NULL
BEGIN
DROP SECURITY POLICY s.policyName
END
GO  
    CREATE SECURITY POLICY s.policyName 
    ADD  FILTER PREDICATE s.policyName(ColA) ON TableA
    WITH (STATE = ON)       
GO

I did try adding an if condition before and after the Filter predicate, but its not working. Any help is really appreciated.

Answer 1

Wouldn't you just want to make sure the table (and column!) exists before creating the policy, rather than making the policy check for the table every time? I don't know that you can drop a table that has a policy associated with it (you certainly can't if the policy function, which has to be schema-bound, references the table), but I think if people can drop tables after the policy is created there is a bigger issue.

IF EXISTS 
(
  SELECT 1 FROM sys.tables AS t
    INNER JOIN sys.columns AS c
    ON t.[object_id] = c.[object_id]
    WHERE t.name = N'TableA'
      -- AND t.[schema_id] = SCHEMA_ID(N's') -- 'dbo', 'other' ???
    AND c.name = N'ColA'
)
BEGIN
    CREATE SECURITY POLICY s.policyName 
    ADD  FILTER PREDICATE s.policyFunctionName(ColA) ON TableA
    WITH (STATE = ON);
END

And of course you can't create the policy in the first place if the table doesn't exist. It will fail with:

Msg 33268, Level 16, State 1
Cannot find the object "TableA" because it does not exist or you do not have permissions.

So you can use the IF EXISTS for nice behavior (you could put a friendly error message in an ELSE ) or you could just wrap in TRY/CATCH and hope for the best.

You might also want to make sure that the function s.policyFunctionName exists for a similar reason (and of course you can't call the policy the same as the function), or just let it fail.

Answer 2

SQL checks if the table exists, true the result is 1, if the table does not exist the result is zero. For Example:

IF EXISTS ( USE TableA
            SELECT 1 
            FROM INFORMATION_SCHEMA.TABLES 
            WHERE TABLE_TYPE='VIEW' 
            AND TABLE_NAME='TableA') 
   SELECT 1 AS res ELSE SELECT 0 AS res;
  
IF OBJECT_ID ('TableA.s.policyName') IS NOT NULL 
   SELECT 1 AS res ELSE SELECT 0 AS res;